1. Field of the Invention
The present invention concerns digital radio communications and, more particularly, a multimode radio communication terminal able to operate in at least two different radio communication systems using relatively close frequency bands, i.e. frequency bands that can be processed by the same technological means.
2. Description of the Prior Art
The invention may be applied in a dual mode GSM/DECT terminal, for example. The GSM system uses the frequency bands 890-916 MHz and 935-960 MHz for the two transmission directions and the DECT system uses the frequency band 1880-1900 MHz.
In the transmitter of a radio communication terminal a local oscillator produces a signal at the frequency of a transmit channel. In the receiver of a radio communication terminal the local oscillator produces a signal at the frequency of a receive channel. The signal at the transmit channel frequency, or transmit local oscillator signal, is modulated by the signal to be transmitted to produce a transmit signal. The signal at the receive channel frequency, or receive local oscillator frequency, is used to demodulate a receive signal carrying the information to be received. In the case of direct demodulation the receive local oscillator signal is combined vectorially with the receive signal picked up by the antenna to obtain a baseband information signal.
In different calls a terminal must be tuned to transmit and receive channels that are each different each time, depending on the local availability of the system in which it is used. The transmit and receive local oscillator signals are therefore provided by at least one source that can be tuned to the channels of the system. Since, in the digital radio communication systems discussed here, transmission takes place in a temporal frame with transmission and reception separated in time, a single tunable source is sufficient.
In the case of a dual mode terminal, although the conventional solutions require either a single tunable source with a much greater bandwidth or two separate tunable sources, one for each radio communication system, using a single tunable source of limited bandwidth has already been considered, as described in patent document FR-A-93 07 775. The principle adopted is firstly to achieve concordance of the frequency bands of the two systems and then to tune the terminal to a transmit channel or a receive channel using a single tunable source the tuning frequency band of which is more or less that of one only of the systems.
However, this requires an additional intermediate demodulator system in the receiver converting the receive band of one of the systems for concordance with the receive band of the other system.
Accordingly, an object of the present invention is a multimode radio communication terminal including, as previously, a tunable source of transmit and/or receive local oscillator signals, for transmitting and/or receiving one of the channels of different frequency bands in at least two radio communication systems but that does not require any frequency band conversion beforehand.